When the heat hits Deniliquin, residents are spoilt for cool relief - the Edward River is typically high in summer to supply nearby irrigators and slake the thirst of distant Adelaide.

Just as well. The Riverina town notched up a blistering 44.3 degrees on Tuesday - one of the state's hottest - and there are three more days of such extreme weather to come as the nation's second big heatwave for 2014 drifts east towards Sydney.

Melbourne: Copped 42.8 degrees on Tuesday and can expect similar temperatures in the 40s over the next three days.

Halfway between Adelaide and Sydney, Deniliquin sits on a branch of the Murray. With fewer bends, the Edward is a quicker way to channel water to Adelaide than the Murray itself, Deniliquin Council general manager Des Bilske said.

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Coming the other way, though, is heat – and lots of it. “Generally with the weather, what comes out of Adelaide comes through to Deniliquin and heads to Sydney,” Mr Bilske said.

Sydneysiders – at least those not living near the coast - should be worried.

Adelaide's 45.1-degree scorcher on Tuesday was the city's fourth hottest day on record - and the records go back to 1888, Bureau of Meteorology climatologist Blair Trewin said. Those blistering conditions are set to get worse, with 46 predicted for the South Australian capital on Thursday. Adelaide's record high of 46.1 degrees, set on 12 January 1939, may be broken.

Sydney will dodge the worst of the heat because sea breezes should be strong enough to keep a lid on temperatures, but western parts of the city won't be so lucky.

The temperature should remain between 29 and 31 degrees until Saturday on the coast, with a maximum of 30 forecast on Wednesday, the bureau says. But in the west, temperatures are set to hit 37 from Wednesday to Friday and 39 on Saturday.

Early hot spots on Wednesday include Ivanhoe Airport in western NSW, with 33.5 degrees reached by 8.35am. Yarram Airport in Victoria had recorded 36.8 degrees by just after 8.30am, and Parafield in South Australia was at 34.8 by 8am, local time.

Open outlook

Melbourne, where the Australian Open is in full swing, copped 42.8 degrees on Tuesday and can expect similar temperatures in the 40s over the next three days, before a cool change on Friday afternoon.

The city experienced a more extreme three-day spell in January 2009, before the Black Saturday fires, but it has had only one other stretch of four days above 40 degrees – back in 1908, Dr Trewin said.

Edward River: Residents of Deniliquin will be thankful for the river's cool relief after the town notched up a blistering 44.3 degrees on Tuesday. Photo: Quentin Jones

Melbourne's overnight temperature dropped to only 28.6 degees at 6am on Wednesday, before starting to climb again. It was still above 30 degrees at 5am, making for a particularly uncomfortable night for the city's residents.

Campers at Victoria's Wilson's Promontory won't have had much use for sleeping bags. The tourst site was the state's warmest early on Wednesday at 33.8 degrees.

Tuesday's heat saw some records broken, mostly at shorter-term site in south-eastern SA, with Keith's 46.9 and Naracoorte enduring 45.8 degrees, he said. Strahan on Tassie's west coast set a January record for the site with 38.

Heatwave - January 2014

Second heatwave of 2014 heads east. Follow us at http://twitter.com/photosSMH Photo: Paul Jeffers

Fire threat

For much of south-eastern Australia, Friday looks likely to pose the biggest challenge for firefighters. A cool change will bring squally gusts as winds shift around from the north to the south. Wind strengths may reach 35-50 km/h for areas such as Melbourne, the bureau said.

“Friday could be the most severe day for fire danger,” Lindsay Gilbert, a senior meteorologist from the bureau's National Operations Centre, said.

Overnight, lightning strikes in Victoria have started more than 250 new fires in the state's north-west and central regions, AAP reported.

Back in Deniliquin, the local swimming pool was gearing up for a surge of visitors late on Tuesday, as families with children too young to ski or play in the river wait for the heat to ease before taking a dip.